CM1200 Modem having to use 2 networking cables for both internet and the configuration page
Since I bought this modem at the end of year 2019, The modem works, but I have a few questions. I guess it's appropriate to have one discussion per question:
I have only been able to access the CM1200 modem's 192.168.100.1 configuration page by either having only the computer directly connected with a RJ45 CAT5e cable but with coax disconnected, or by having one RJ45 port connected to the router's (Asus AX11000) WAN and the second modem's RJ45 port (I'm using port 4) connected to the router's LAN, or to another networked switch.
Since the modem's IP is not configurable, my network is 192.168.100.XXX, router is .2 and static IPs are set for some devices.
My IPAD and IPhone using dynamic IPs on Wifi have the same results. I tested this also using another router, of the Netgear AC1750 kind (It was a backup router for when I had to RMA the AX11000, but went back to preferring the AX11000 because of the ridiculousness (that I have never seen before and unable to bypass) when I had to go through on the initial set up pages of that AC1750).
I have been tolerating having to plug 2 networking RJ45 cables into one modem and one router to access both internet and modem's configuration page, but I'm wondering if this is normal. I have found very little information on this specific topic, and read one other post here that suggest resetting the CM1200 modem to factory defaults with the coax cable disconnected solved the problem, but for me the problem continues.
I have set the modem to factory defaults many times already due to my ISP (Cox) crashing all the time. I find that after they fix their issues, I have to reset the modem to factory defaults to have stable internet again, until the next ISP outage).
Observations while trying to access the CM1200 modem at 192.168.100.1 using only one networking RJ45 cable connected to a router's WAN: Router get's an external IP from ISP's DHCP giving us internet, but I no longer have access to the modem's page. 192.168.100.1 times-out like any other non-working web page.
Upon connecting a second CM1200 modem port to the router's LAN, I immediately have access to the modem, and internet.
Re: CM1200 Modem having to use 2 networking cables for both internet and the configuration page
Very bad idea to operate the WAN (resp. the related administration subnet) and the NATed LAN on the very same IP subnet.
If you have your LAN say on the common 192.168.1.0/24 (255.255.255.0) you could always reach the CM1200 management IP 192.168.100.1 passing the WAN router / NAT router.
The WAN IP for the router (also handled by the ISP) but coming in over the CM1200 is typically a public IPv4 address (rarely a carrier grade NAT or private IP subnet). Said that, thre is no need to operate the LAN behind your NAT router "just" to manage the CM1200 - and on the very same IP subnet. The reason is simple: The router requires two different subnets - otherwise it can't route.
Even worse: This second connection bypassing the NAT router will cause many rnadom issues - I don't want to think about it, so could a computer on the LAN receive a public IP address from the ISP, and so on - so it's fully exposed, while not able to talk to other systems on your home [W]LAN.
